This invention relates to an electronic device having a frequency dependent signal transfer characteristic. In particular, it relates to an electronic device which is electrically coupled to at least one SAW transducer.
SAW technology is increasingly finding more and more applications in the electronics and RF art. Due to the fact that SAW wavelengths are typically 10.sup.5 times shorter than those of electromagnetic waves, SAW technology is finding particular applications where miniaturization is important or desirable. One such application is the use of SAW filters in radio telephones, where the typically small size and weight of SAW filters is highly advantageous over conventional technologies, such as ceramic filters, lumped element filters or the like. It is a requirement of such filters that they have low-losses, typically insertion losses of 1.about.3dB.
A typical example of a conventional low-loss SAW filter is a transversal SAW filter in which SAW energy is transferred between two spaced apart interdigital transducers (IDTs). The IDTs each comprise two sets of electrode fingers which are formed on the surface of a piezoelectric substrate. The fingers in each set are typically all electrically connected together and are interleaved (interdigitated) with the electrode fingers of the other set. In a transversal SAW filter, electromagnetic energy is converted into SAW energy by coupling the electrostatic field pattern of an input IDT to a SAW by the piezoelectric effect.
A problem encountered with both low-loss SAW filters, and filters utilising conventional technology is that unacceptably high side lobes exist in the stopbands of the filters. Hitherto, this problem has been addressed by cascading identical filters or filters having identical or slightly different frequency transfer characteristics. However, such an approach typically results in complicated filter design, requires space to accommodate the extra components or extra tracks comprising the cascade of filters thereby mitigating against miniaturization, increases insertion loss and reduces the passband of the composite filter relative to the single filter whose characteristics it was desired to improve.
A further problem with SAW filters and transversal SAW filters in particular is that their maximum input power is limited due to the mechanical vibration caused by large amplitude SAWs degrading the IDT electrode fingers resulting in lower performance from the filter. Additionaly, conventional filters have relatively high losses, typically greater than 10 dB for transversal type SAW filters. Similar problems occur with SAW resonator-type filters.
It is known that SAW elements can be conceptually modelled and used as electrical impedance elements. Modelling and using a SAW resonator as an impedance element is possible because a SAW element such as a SAW resonator has an electrical impedance which is, in part, dependent on the electro-acoustic interaction of the electrode fingers of the SAW resonator with the mechanical vibration of the SAW. Near to the center frequency of the SAW element (i.e. the frequency at which the separation of adjacent fngers is .sup..lambda. /2) it has a maximum electrical admittance and a minimum electri.cal admittance. These are respectively the electrical resonant and anti-resonant frequencies of the SAW element. When large changes in electrical impedance are desired the electro-acoustic interaction must be high. Thus, SAW elements with a large number of electrode finger pairs are used. Reflectors can be placed at the ends of the SAW element to inhibit energy losses, thereby creating a resonator-type structure. Alternatively, SAW elements can be used which only have large numbers of electrode fingers since they exhibit electrical properties similar to SAW resonator-type structures. Since the SAW resonators in the known filter are utilized primarily as lumped impedances, it is convenient to term them SAW impedance elements. The term SAW impedance elements will hereinafter be used when referring to any SAW element (IDT SAW resonator or otherwise) which is being particularly used for its electrical impedance properties.
In the foregoing an individual SAW resonator can be modelled as a lumped impedance element connected in series, and a conventional capacitance (static capacitance C.sub.ST) connected in parallel between two ports of the SAW resonator. The static capacitance is due, inter alia, to the capacitance between electrodes of the SAW resonator, between electrodes of the SAW resonator and a ground plane on the substrate, and the resonator to resonator coupling pattern and the ground plane if there are more than one resonator on the substrate.